Babystep1: Difference between revisions

The following code is the smallest possible example of booting code from a floppy.

 

; boot.asm

hang:

 

times 512-($-$$) db 0

The CPU starts in real mode and the BIOS loads this code at address 0000:7c00. The "times 512..." stuff is NASM's way of saying fill up 512 bytes with zeros. And partcopy is going to expect that (200 in Hex = 512 in Decimal). Change it and you'll see Partcopy choke.

 

Often, you will see a so-called boot signature (0xAA55) at the end. Older versions of [[BIOS|BIOSes]] looked for this in order to identify a boot sector on a disk. It is evidentially unnecessary nowadays. If it's needed, the last line would be replaced with (or some version of it)

; boot.asm

hang:

db 0x55

db 0xAA

 

But the thing I'd really like to point out is how once you've booted, and the cursor is happily blinking on a blank screen, you might notice two things. One is that the floppy motor will turn off and the other is that you can press Ctrl-Alt-Del to reboot. The point is that interrupts (such as INT 0x09) as still being generated.

 

For kicks try clearing the interrupts flag:

;boot.asm

cli

db 0x55

db 0xAA

You may notice that the floppy motor doesn't turn off and you can't reboot with Ctrl-Alt-Del.

 

 

===Windows===

===Unix===

 

== References ==